High contrast cathode ray tube



Feb H, 1969 E. ATTI v 3,427,490

HIGH CONTRAST CATHODE RAY TUBE Filed Feb. 15, 1967 c?, N

FG l

FIG. 3.

FIG. 2.

WITNESSES'. INVENTOR M Eros Aii @WUQ BYMKQ ATTORNEY United States Patent() 7 Claims ABSTRACT F THE DISCLOSURE A high contrast cathode ray tube in which a secondary electron suppressor structure is provided for directing and thereby preventing secondary electron bombardment of the display screen due to primary electrons striking regions other than display screen.

This invention relates to a cathode ray tube and more particularly to means -for improving the contrast of cathode ray display devices.

High contrast in display devices is a very important image characteristic. Losses in contrast may occur on account of light reflections inside the tube or its faceplate. It may also be caused by external ambient illumination of the display device. Many proposed solutions to the external ambient illumination problem can be found in the art. Among the most common means to improve image contrast are metalized screens, coatings on the faceplate and darkened bulb interiors and faceplates.

Another source of background lighting within a cathode ray display tube is due to secondary electrons generated within the envelope. These secondaries strike the phosphor and cause emission of light degrading the image, A common source of secondary electrons within the cathode ray tube is due to the overscanning of the electron beam so as to cover a region Igreater than the active display area. In overscanning with the electron beam, the beam normally impinges on the glass wall of the envelope or conductive coatings provided on the envelope.

The emission of these secondary electrons from the point of impact of the primary beam upon the envelope wall is in all directions. Those which strike the phosphor screen have the possibility, depending upon their energy, of exciting the fluorescent material to cause emission of undesired light. The amount of this spurious light generated by these secondaries depends upon the characteristics of the wall surface, the energy distribution of the secondary electrons and, of course, the number of secondary electrons.

It is accordingly an object of this invention to provide an improved cathode ray tube in which the bombardment of the screen by secondary electrons is substantially reduced.

It is another object of this invention to provide an improved cathode ray tube in which practically none of the secondary electrons, including the backscattered or reliected primaries, originating at the point of impact of the primary beam on target are directed toward the screen or target of the cathode ray tube.

Briefly, the present invention accomplishes the above cited objects by providing a secondary emission suppression structure for intercepting the primary electrons in the electron beam which would strike areas of the cathode ray tube outside of screen area, directing the secondary electrons generated at this structure away from the screen and thus preventing the secondary electrons from striking the screen.

These and other objects and advantages of the present invention will become more apparent when considered in 3,427,490 Patented Feb. 11, 1969 'ice View of the following detailed descriptions and drawings, in which:

FIGURE 1 illustrates a cathode ray tube partly in section incorporating the teachings of this invention;

F-IG. 2 is a sectional view of a modified secondary electron suppressor as shown in FIG. 1; and

FIG. 3 is a sectional view of a modied secondary electron suppressor as shown in FIG. 1;

Referring to FIG. 1, there is illustrated a high contrast cathode ray tube. The tube includes an envelope 12 having a tubular neck portion 14 and a tubular display portion 16 having a larger diameter than the neck portion 14. An interconnection part 18 connects the two tubular members 14 and 16. A faceplate portion 20 closes off the opposite end of the display section 16 and the opposite end of the neck portion 22 is closed off by a base member 22.

Positioned within the neck portion 14 of the envelope 12 is a suitable electron gun 24 for generating an electron beam. The electron gun 24 may be of any suitable type. The electron gun 24 shown in FIG. 1 includes a control grid 26, a first anode 28, a second anode consisting of electrodes '30` and 32 and a focusing electrode 34 positioned intermediate the electrodes 30 and 32. The electrodes 30 and 32 are connected together electrically and operate at the same potential. An electrically conductive coating 36 is provided on the inner surface of the sections 16 and 18 and extends into the neck portion 14. Voltage is supplied to the electrodes 30 and 32 by means of contact members (not shown) to this conductive coating 36. Potential is normally applied to the conductive coating 36 by means of an anode button 38. A cathode (not shown) is provided within the control grid 26 to serve as the source of electrons and the electrons thus provided are accelerated and focused by the electron gun electrodes which follow the control grid 26. Suitable voltages are applied to these electrodes in a well known manner by means of leads 39 provided in the base 22 and by the anode button 38.

Deflection of the electron beam generated by the electron gun 24 is provided by means of a suitable deection means such as yoke 40 arranged around the neck portion 14. The deflection means, such as yoke 40, is fed with appropriate currents in a well known manner to deect the electron beam over the display area provided on the faceplate 20'. The faceplate 20 is provided with a luminescent coating 42 of a suitable phosphor that emits light in response to electron bombardment. An electron energy filter 44 may be provided on the inner exposed surface of the phosphor layer 42 to filter out or stop those secondary electrons with energies below a given level and thus prevent them from exciting the luminescent coating 42. The lter 44 may be a thin lm of a suitable material such as aluminum having a thickness of about to 1000 angstroms. The lm 44 may even be thinner for displays operating at low screen voltages. The film 44 is normally electrically connected to the coating 36 and operates at the same potential.

A rst secondary electron suppressor electrode `50 is provided within the neck portion 14 and may extend into the display portion 16 as shown in FIG. l. In FIG. l, the electron suppressor electrode 50 is shown to be tubular in shape. It may be of rectangular or circular or any other cross section depending on the active area of the screen 42. The member 50 is disposed about the longitudinal axis of the tube. It will intercept any electrons of the primary beam traveling along a path such as indicated at `52 in FIG. l. The length and position of the member 50 determines the electrons intercepted. The secondary electron suppressor electrode 50` is mounted within the neck portion 14 by any suitable means such as post members 54 secured to the neck portion 14. These post members 54 also provide means of applying a potential similar to that applied to the coating 36 to the secondary electron suppressor 50. The potential applied to the suppressor electrode 50 may be the same as that applied to 36. The suppressor electrode 50 made of or provided with an intercepting surface of a material of low atomic number such as carbon, aluminum etc. The inner surface of the suppressor electrode 50 is provided with a plurality of axially spaced projecting iins 56 each having thin or sharp protruding edges 57. Each of the fins 56 provides an intercepting surface 59.

The secondary electron suppressor electrode V501 may be provided directly on the inner surface of the neck portion 14 in the form of a coating of a suitable material such as carbon black or Aquadag with the fins 56 provided directly in the glass wall. The electron suppressor may also be a helix member 70 such as illustrated in FIG. 3. The helix member 70 may be separately wound and fitted into the neck portion or a cylindrical member supported, for instance, by the top end of the gun. This helix member 70 may be a simple edge wound ribbon sharpened on the inner edge. A suitable material may be aluminum. The ribbon may be also of higher melting temperature metals such as stainless steel.

In operation, a portion of the primary electrons from the beam, such as those along line 52, strike the surface of a fin 56 facing the electron gun 24. The primary electrons striking fin 56 produce secondaries which are emitted backward toward the gun 24.1n their flight away from the impacted iin surfaces 56, the secondaries, which also include reflected primaries, may strike a surface of a fin 56 facing the display screen 42 or other parts of the gun or envelope with the possibility of producing a second `generation of secondaries. In turn, these secondaries may strike a third time. It is well known that with regard to secondary electron emission that the majority of the secondaries have an energy of less than 20 volt-electron or 20 v.e. and that the percentage of secondaries fromed by reected primaries in the range of energy of 98 to 100 percent of the primary electron energy increases with the atomic number. By proper spacing and inclination of the ns S6, one can assure that practically all the first generation secondaries, including the reflected or backscattered primaries, will be emitted in the direction of the electron gun 24 rather than the screen 42. None of the secondary electrons is thus in a position to excite the screen 42. A negligible number of electrons will be intercepted by the sharp or thin edges 57 of the fins 56 and generate secondary electrons a part of which may travel to the screen.

The modification illustrated in FIG. 2 illustrates a cross sectional view of another embodiment of secondary emitter suppressor surfaces that can be employed within the secondary electron suppressor 50. The electron suppressor 50 can also be formed by means of grooves cut into the wall or into a coating provided on the wall and cut in a helicoidal shape similar to screw threads if so desired. The suppressor surface may be of carbon, aluminum or coatings of other low atomic number material. Suitable support members could be formed of an insulator such as ceramic coated with a layer of electrically conductive material such as a carbon black coating. This type of structure will not interfere with the magnetic deflection fields of yoke 40.

Positioned in the display region 16 of the tube is a second secondary electron suppressor 60 which may be of similar materials and structure and supported in similar ways as described with respect to the secondary electron suppressor 50. The member 60 is again tubular in shape and may be designed on the same basis as the element 50'. Again FIGURES 2 and 3 are additional embodiments that might be utilized for the member 60. The member 60 is again normally connected to the electrically conductive coating 36 and would intercept a primary electron beam following the path A62. The function of the secondary electron suppressor i60 with regard to the electron beam 62 Iwould again be similar to that described with respect to the suppressor 50.

For economic reasons one of the secondary electron Suppressors 50 or 60` could be omitted from the tube and of course adjustment of a single member might be necessary in order to obtain maximum eiciency. The spurious light output would be reduced with a single suppressor but the results would not be as good as that found with the use of two Suppressors. This holds true even with precisely aligned and centered beams which would allow the beam to graze the element 50 at the screen edge. In fact, due to the diameter of the beam at the exit aperture, there will be a certain bulb length swept by the beam before the beam is completely intercepted by the element 50u If it is acceptable to have a narrow ring surrounding the screen where the brightness may gradually fall off as the beam approaches the screen edge, then the structure wherein a single element 50 is utilized in a position slightly forward of the position shown in FIG. 1 would be fully effective.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoin-g description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. A cathode ray tube comprising a display screen of a given area responsive to electron bombardment, means for generating and directing an electron beam onto said screen, means for deecting said electron beam to scan a raster larger than the area of said display screen and means positioned between said means for generating and directing said electron beam and said display screen for intercepting a portion of said electron 'beam raster directed onto areas other than said display screen, said means including a plurality of intercepting surfaces spaced along the axis of said tube.

2. A cathode ray tube as described in claim 1 in which said interception surfaces, for suppressing secondary emission from the intercepting surface in a direction toward said display screen, are of a low atomic number material.

3. A cathode ray tube as described in claim 1 in which said intercepting surfaces terminate with sharp edges.

4. A cathode ray tube as described in claim 1 in which said display screen is provided with a layer of material for preventing the excitation of said display screen in response to electrons of a lower energy than said electrons in said electron beam.

5. A cathode ray tube as described in claim 1 in which said electron beam interception means is a tubular member having an irregular surface on the inner surface thereof.

6. A cathode ray tube as described in claim 1 in which said intercepting means is a coating on the inner surface of said tube.

7. A cathode ray tube as described in claim 1 in which said intercepting means is a helical member adjacent the inner surface of said tube.

References Cited UNITED STATES PATENTS 2,487,078 11/1949 Sloan 313-106 X 2,916,664 12/ 1959 Sternglass 313-106 X 3,054,019 9/1962 =Hendry 315-11 RODNEY D. BENNETT, Primary Examiner. JEFFREY P. MORRIS, Assistant Examiner.

U.S. Cl. X.R 315--12 

